1. Field of the Invention
The present invention generally relates to computer systems. More particularly, the present invention relates to computer graphics and a graphical image display system that use a visual server to generate and transmit images to a client.
2. Description of the Related Art
Computer generated images are created in a series of several steps: modeling, or describing the objects in mathematical terms; animation, or describing how the objects move over time; and rendering, or determined which of the images defined by the modeling and animation programs are visible on the screen for every frame, assigning color to those images, and drawing them. Of these, rendering is the most difficult as it is the most processing intensive action. Because the generation of images is both processing and memory intensive, a client computer often relies upon other processors and memories, such as a server, to assist in generating the data to render new images. In the existing network model, storage and caching of content and applications can occur on a centralized server, but the actual software applications execute on a system in the client user space. The network thus allows the server to supply image processing resources beyond the constraints of a remote terminal, while taking advantage of client resources for certain tasks. Several existing systems utilize the client-server model to process and render images at a client display with the assistance of server resources.
In methods of client-server graphics that employ image transmission, the server unit requires a relatively high bandwidth connection even if real-time image compression and decompression is employed. Even in geometric replication methods, such as VRML, which generally have a lower bandwidth requirement, the image processing does not decrease the storage or processing requirements of the client. In these systems, the client must be capable of 30 rendering the complete replicated database in real time, and the server does not provide true image generation services. Rather, the server functions primarily as a database server. For example, in a client-server method based on a hybrid image transmission and geometry transmission approach, the server unit maintains a low level-of-detail database and a high level of detail database. For each frame of a real-time image stream, images are generated from both the low level-of-detail and high level-of-detail database. The difference between the two images is computed using known methods of image processing. The client unit contains only a low level-of-detail geometric database and renders this database for each frame, and the difference image computed on the sever is compressed and transmitted to the client in real-time which decompresses and composites it together with the image generated by the client from the low level-of-detail database. The result is a high level-of-detail, high quality image that requires a relatively low transmission bandwidth to transmit the relatively low information difference image. Such method also requires relatively limited client storage and processing capabilities. While this method can decrease the computational load of the client and reduce the communication costs, it requires both real-time image compression and decompression and further requires that special image compositing be added to the pipeline. In addition, the entire low level-of-detail database must be stored and processed by the client.
Another known method of client-server image processing is transmitting a transformed and compressed low level-of-detail representation of the geometric database in screen-space representation from the client to the server. The low level-of-detail geometric primitives are transmitted for every frame, and such action increases the required connection bandwidth. Only the primitives that are actually visible for each frame need to be transmitted in this approach. Moreover, since primitives visible in a current frame are likely to be visible in a subsequent frame, the repeated transmission of primitives that have been transformed to image-space representation is an inefficient use of available bandwidth. Any processing to identify repetitive primitives requires further client resources, which hampers the image displaying process.
A further approach to distributed client-server image generation is based on demand-driven geometry transmission to the client. In this method, the server determines and periodically updates a list of potentially visible primitives for each client using a spherical clipping volume around a viewpoint. The result is then compared to a list of primitives previously transmitted to the corresponding client and only those potentially visible primitives that have not been previously transmitted are sent to the client. However, this method reduces communication cost by limiting transmission to those primitives that have become potentially visible. The client replaces primitives in the client display list when they are no longer included in the spherical clipping volume, and thus, the storage and computing requirements of the client are limited to only those primitives in the potentially visible set. However, as this method uses a limited inclusion volume, a major disadvantage is that distant primitives can be arbitrarily excluded from the potentially visible data set. Further, the use of a spherical inclusion volume results in the inclusion and transmission of a large number of geometric primitives that are not visible in the current frame and are unlikely to be visible in upcoming frames, an example being primitives in the inclusion sphere behind the viewpoint. As a result, the demand-driven geometry transmission makes inefficient use of available transmission bandwidth and available client storage and computing resources, and the client resources are taxed because the client must compute removal of primitives by clipping to the inclusion volume, and implement display list replacement and compaction protocols.
Client graphic image display processing capabilities are even further taxed in the display of 3-dimensional images. In existing clients, the implementation of a system delivering 3D graphics requires graphics hardware at the client, which presents several problems. The additional hardware increases the cost of the client hardware as the graphics hardware must be incorporated and integrated therewith. Further, the software and hardware used to generate 3D images is in constant flux, and the system must be continually upgraded, creating an additional expense and a logistical burden for the client. Moreover, the remote hardware impedes the central maintenance and coordination of configurations of client software, which is an important capability and critical to the product viability of many applications.
Some clients use separate 3D hardware component to assist in processing the images, but with the constant change of graphics languages and protocols, the hardware becomes obsolete rather quickly, which adversely impacts the value of the client. Furthermore, the use of programmable graphics hardware within the client system is difficult because the client resources are inherently static and limited.
Therefore, for reasons of cost, size, and power consumption, sophisticated three dimensional graphics are not available on common consumer client devices such as personal digital assistants (PDAs) mobile telephones and set-top boxes used to decode cable and satellite television signals. Consequently, it would be advantageous to display complex three dimensional graphics, such as those used by games, on these consumer client devices.
Accordingly, it is to the provision of such an improved system and method of displaying images on a client consumer device utilizing the resources of a visual server that the present invention is primarily directed.